Turbofan gas turbine engines for powering aircraft conventionally comprise a core engine, which drives a fan. The fan comprises a number of radially extending fan blades mounted on a fan rotor enclosed by a generally cylindrical fan casing.
The fan casing is provided with a hook axially upstream of the tips of the fan blades to prevent upstream movement of a detached blade. Upstream of the casing hook there is provided front acoustic panels that form the annulus profile forward of the casing hook.
Conventionally, the acoustic panels are either bolted to the casing or, where the casing for stress reasons cannot sustain the presence of radial holes, held in place by an ‘o’ ring and bracket arrangement.
Such an arrangement is disclosed in GB 2407344 (incorporated herein). A turbofan gas turbine engine 10, as shown in FIG. 1, comprises in flow series an intake 12, a fan section 14, a compressor section 16, a combustor section 18, a turbine section 20 and an exhaust 22. The turbine section 20 comprises one or more turbines arranged to drive one or more compressors in the compressor section 16 via shafts. The turbine section also comprises a turbine to drive the fan section 14 via the shaft. The fan section 14 comprises a fan duct 24 defined partially by a fan casing 26. The fan duct 24 has an outlet 28 at its axially downstream end. The fan casing 26 is secured to the core engine casing 36 by a plurality of radially extending fan outlet guide vanes 30. The fan casing 26 surrounds a fan rotor 32, which carries a plurality of circumferentially spaced radially extending fan blades 34. The fan rotor 32 and fan blades 34 rotate about the axis X of the gas turbine engine 10, substantially in a plane Y perpendicular to the axis X. The fan casing 26 also comprises a fan blade containment assembly 38, which is arranged substantially in the plane Y of the fan blades 34. The fan blades 34 have a leading edge 33, a trailing edge 35 and a tip 37.
The fan casing 26 and fan blade containment assembly 38 is shown more clearly in FIGS. 2 to 4. The fan blade containment assembly 38 comprises a metal cylindrical, or frustoconical, casing 40. The metal casing comprises an upstream flange 42 by which the fan blade containment assembly 38 is connected to a flange on an intake assembly of the fan casing. The metal casing comprises a downstream flange 44 by which the fan blade containment assembly 38 is connected to a flange on a rear portion of the fan casing 26. The metal casing 40 comprises an annular T-shaped member, or hook, 54 that is positioned axially upstream of the leading edge 33 of the tip 37 of the fan blade 34. The annular member 54 comprises a first portion 53 which extends in a radially inwardly direction from the metal casing 40, a second portion 55 which extends in an axially downstream direction from the radially inner end 53A of the first portion 53 of the annular member 54 towards the tip 37 of the fan blade 34 and a third portion 57 which extends in an axially upstream direction from the radially inner end 53A of the first portion 53 of the annular member 54 towards the intake assembly 46. The annular member 54 is substantially in a plane perpendicular to the axis X of the gas turbine engine 10, upstream of the most upstream point of the leading edge 33 of the fan blades 34.
A liner 70 is provided radially within the metal casing 40 and there are two types of liner 70, an acoustic liner 72 to reduce noise and a fan blade track panel 71 arranged around the fan blades 34 to form an abradable seal.
The acoustic liner generally comprises a plurality of acoustic panels 72, which are arranged circumferentially and axially along the inner surface of the metal casing 40. Each acoustic panel 72 comprises a perforated skin 74 and a structure 76 to form an acoustic treatment structure. The perforated skin 74 has a plurality of perforations 78.
The perforated skin 74 of each acoustic panel 72 comprises aluminium, titanium or composite material, for example fibre reinforced plastic e.g. glass fibre reinforced epoxy resin. The structure 76 comprises a honeycomb structure, for example an aluminium honeycomb, a titanium honeycomb, a composite material honeycomb, a resin impregnated paper honeycomb or other suitable honeycomb.
The composite material honeycomb may comprise fibre reinforced plastic e.g. glass fibre reinforced epoxy resin. The panels of the liner 70 are secured to the metal casing 40 by various means. A circumferentially arranged set of acoustic panels 72 is arranged upstream of the annular member 54. The acoustic panels 72 are secured to the metal casing 40 by an axially extending tenon 80 on the axially downstream end 82 of each acoustic panel 72 and an axially extending member 84 on the axially upstream end 86 of each acoustic panel 72, as shown more clearly in FIGS. 4 and 5. The axially extending members or tenons 80 extend in an axially downstream direction radially outward of the axially extending third portion 57 of the annular member 54 and thus secure the downstream ends 82 of the acoustic panels 72A.
In the prior art an annular resilient sealing member, for example a rubber, or silicone, sealing member, 92 is located in an annular groove 90 on the radially outer surface 88 of the third portion 57 of the annular member 54. The annular resilient sealing member 92 thus biases the axially extending members or tenons 80 and the axially downstream ends 82 of the acoustic panels 72A radially outwardly into contact with the inner surface of the casing 40.
The flange 42 of the casing 40 has a plurality of circumferentially spaced recesses 94 on the face arranged to abut the flange 48 of the intake assembly 46. A plurality of L-shaped brackets 96 are provided and each L-shaped bracket 96 fits in one of the recesses 94 on the face of the flange 42 and each L-shaped bracket 96 is arranged flush with the face of the flange 42. Each L-shaped bracket 96 comprises a radially extending portion 98 and an axially extending portion 100. The axially extending portion 100 extends in an axially downstream direction and the axially extending members 84 extend in an axially upstream direction radially outwardly of the axially extending portions 100 of the L-shaped brackets 96 to secure the upstream ends 86 of the acoustic panels 72.
Each L-shaped bracket 96 has a resilient member, for example a rubber, or silicone, resilient member 102 located on the radially outer surface of the axially extending portion 100 of the L-shaped bracket 96. The resilient members 102 thus bias the axially extending members 84 and the axially upstream ends 86 of the acoustic panels 72 radially outwardly into contact with the inner surface of the casing 40. The radially extending portion 98 of each L-shaped bracket 96 has a countersunk aperture 104 arranged coaxially with one of a plurality of circumferentially arranged apertures 106 in the flange 42 of the casing 40.
The L-shaped brackets 96 are secured to the flange 42 of the casing 40 by countersunk headed bolts, or screws, 108 passing through the apertures 104 and 106 in the L-shaped brackets 96 and flange 42 and respective nuts 110.
The acoustic panels 72 are installed by moving them in an axially downstream direction so that the axially extending members or tenons 80 locate on the radially outer surface 88 of the third portion 57 of the annular member 54. The L-shaped brackets 96 are located in the recesses 94 in the surface of the flange 42 and the axially extending portions are positioned radially inwardly of the axially extending members 84 so that the axially extending members 84 locate on the axially extending portions 100 of the L shaped brackets 96. The bolts, or screws, 108 are inserted through the apertures 104 and 106 in the L-shaped brackets 96 and flange 42 and threaded into the nuts 110.
The acoustic panels 72 are removed by unthreading the s bolts, or screws, 108 from the nuts 110, removing the bolts 108 from the apertures 104 and 106 and removing the L-shaped brackets 96 from the recesses 94 in the flange 42.
The acoustic panels 72 are removed by moving them in an axially upstream direction so that the axially extending members or tenons 80 no longer locate on the radially outer surface 88 of the third portion 57 of the annular member 54.
Preferably the L-shaped brackets 96 comprise a suitable metal or other suitable material. Other suitable shapes of bracket may be used.
Each acoustic panel 72 may be secured at its axially upstream end 86 by a single L-shaped bracket 96, which extends through a substantial portion, or the full length, of the axially upstream end 86 of each acoustic panel 72 respectively. Alternatively each acoustic panel 72 may be secured at its axially upstream end 86 by a plurality of L-shaped brackets 96 which are spaced apart along the length of the axially upstream end 86 of each acoustic panel 72 respectively.
The advantage of this embodiment is that it allows the acoustic liners to be easily installed, inserted, and removed from the casing. However, gas turbine engines are often built in a vertical attitude. The annular resilient sealing member 92 is an ‘o’ ring that can be expensive and can be up to 10 m in length. The flexibility of the ‘o’ ring means that it tends to want to fall out of the machined groove during assembly. Two people are therefore required to provide a satisfactory build.
Additionally, the axial position of each acoustic panel can vary and frettage may occur between the rail and the casing forward of the ‘o’ ring.